Cellulose acylate film has been used for a variety of liquid crystal display devices, as optical components thereof such as a support of optical compensation film, a protective film for polarizing plate, and so forth.
With recent expanding applications of liquid crystal display devices, there has been a growing demand for large-sized, high-definition applications typically for television sets, and quality levels required for the polarizing plate and the polarizing plate protective film have been becoming more stringent. In particular, the large-sized and high-definition liquid crystal display devices have also been required to operate under various environments which have not been encountered before, for example as electronic signage or other outdoor use, even under severe high-temperature environments. The polarizing plate of the liquid crystal display devices, having been widely used, is configured by a polarizer, using polyvinyl alcohol (PVA) and iodine, held between the polarizing plate protective films typically composed of a cellulose acylate film. The polarizer using PVA and iodine has been suffering from a drawback that the performance thereof is likely to degrade under severe high-temperature environment, more specifically under high temperatures and high humidity. Accordingly, there has been a strong demand for further improvement.
Aimed at solving the durability of polarizer under such conditions, Patent Literature 1, for example, proposes a protective film for the polarizing plate, which contains a cellulosic ester film, and at least one species of plasticizer selected from rosin, rosin derivative, novolac-type epoxy resin, ketone resin, and toluenesulfonamide resin. Patent Literature 2 proposes use of a cellulose acrylate film, which was formed by adding an acrylic polymer having a molecular weight of 500 to 5,000 to the cellulosic ester, as the protective film for the polarizing plate.
However, when the protective films proposed by Patent Literature 1 and Patent Literature 2 are used under high temperatures and high humidity over a long time, the performance of the polarizer becomes degraded with time, proving that they were insufficient in terms of keeping necessary performances of the polarizer and further improvement would be necessary. Another problem was that the plasticizer in Patent Literature 1 and the acrylic polymer in Patent Literature 2 were less compatible with the cellulosic ester only to give a weak interaction, so that not only an effect of addition could not fully developed, but also the film was degraded in translucency (haze).